Recently, demands for semiconductor lasers have been increasing in areas such as optical communication, laser printers, and optical disks, and vigorous studies and developments have been made mainly on GaAs type and InP type semiconductor lasers. In the area of optical information processing, a method in which recording and reproduction of information are performed by means of AlGaAs type semiconductor laser light having a wavelength of about 780 nm, in particular, has been put into practice, and the use of such a method has been wide-spread for compact disks or the like. In recent years, however, there is a demand for greater storage capacity in these optical disk apparatuses, and accordingly, demand for realizing a short-wavelength laser has been increasing.
In this respect, the AlGaInP type semiconductor laser is capable of oscillating in the red region of about 630 nm to about 690 nm. At present, the AlGaInP type semiconductor laser can realize the shortest wave-length of light among semiconductor lasers at the practical level. Therefore, the AlGaInP type semiconductor laser is expected to be the next-generation light source for recording mass optical information, replacing the conventional AlGaAs type semiconductor laser.
Upon reproducing an optical disk by employing a semiconductor laser as a light source, an intensity noise occurs due to the return of reflected light from the disk surface or a change in temperature. Such an intensity noise induces the reading error of a signal. Therefore, as the light source for an optical disk, a semiconductor laser having less intensity noise is essential.
Conventionally, in a low-output AlGaAs type semiconductor laser which is used as a light source for a reproduction only optical disk, low noise has been realized by employing the structure such that saturable absorbers are intentionally formed at both sides of a ridge stripe in order to reduce noise. With such a structure, multiple longitudinal modes are achieved. In the case where disturbance occurs due to returning light, a temperature change, or the like, when a semiconductor laser is oscillating in a single-longitudinal mode, an adjacent longitudinal mode starts to oscillate due to a subtle change in a gain peak. The mode which starts oscillating in such a manner competes with the original oscillation mode, thereby causing a noise. On the other hand, if multiple longitudinal modes are realized by using the aforementioned method, changes in intensities for the respective modes are averaged, and no intensity changes due to disturbance occur. Therefore, it becomes possible to obtain stable low-noise characteristics.
Moreover, as another method, Japanese Laid-open Publication No. 63-202083 describes an attempt to realize more stable self-pulsation characteristics. Specifically, a self-pulsation type semiconductor laser is realized by providing a layer capable of absorbing output light.
Furthermore, Japanese Laid-open Publication No. 6-260716 has reported that operating characteristics of the semiconductor laser is improved by substantially equalizing the bandgap of an active layer with the bandgap of an absorbing layer. The aforementioned publication especially discloses the structure in which the energy gap of a strained quantum-well active layer and the bandgap of a strained quantum-well saturable absorbing layer are set to be substantially equal to each other. With such a structure, it is attempted to obtain satisfactory self-pulsation characteristics.
Furthermore, the same structure as that of Japanese Laid-open Publication No. 6-260716 is described in Japanese Laid-open Publication No. 7-22695.